The present invention relates to a method for coupling a shaft with a hub of a spindle motor having a dynamic pressure bearing construction, which motor is a spindle motor used for a disk device, DVD, and CD-ROM of a computer or the like, particularly, a thin-type hard disk drive motor mounted on a portable personal computer such as a note-type personal computer.
Conventional methods for coupling a shaft with a hub include methods such as press-fitting, shrink fitting, and joining by way of adhesives, and welding.
However, in these conventional coupling methods, where a small diameter shaft (for example, 3 mm of outside diameter) is coupled with a thin hub (for example, 1 mm of thickness) of a dynamic pressure bearing type spindle motor of a thin type hard disk device, the respective methods had the following problems.
As is well-known, the press-fitting method has a limit in strength and particularly is weak in shock. In the shrink fitting method, since a shaft is small in diameter, a shrink fitting allowance cannot be secured, and the method has a limit in strength, similarly to the press-fitting. Further, heating is necessary, and the precision after shrink fitting is poor, so that after coupling processing is necessary. Furthermore, it takes much time for heating and cooling, resulting in poor productivity.
In the method using the adhesive, since the length of adhesion is short, the method has a limit in strength and is weak in shock. The extra adhesive forced out from the joined part adversely influences on the performance of a motor. Furthermore, setting of the adhesive requires time and heating, thus lowering the productivity.
In the welding method, the thermal deformation occurs, and particularly, the bend occurs due to the thermal shrinkage, failing to secure the accuracy. Where the material for a shaft comprises a martensite texture of high volume of carbon such as SUS440C, there was a problem such that a crack occurs in the shaft. Further, an expensive machine such as a laser welder was necessary.
As described above, in the conventional methods, it was difficult to achieve the coupling capable of securing the strength such as loose, bending, turning torque or the like necessary for stress occurring in the coupled part between a shaft and a hub by clamping a disk to the hub, and the run out accuracy relative to the surface of the disk on which the shaft and the hub are mounted, and the coupling for giving sealing properties to the coupled part.
The present invention has been accomplished in order to solve the problems noted above with respect to prior art, and provides a method for coupling a small diameter shaft with a thin hub with high strength and high accuracy, and a coupling method for giving high sealing properties to the coupled part.
For achieving the first object, according to the present invention, there is provided a method comprising: press-fitting an outer circumferential part having an annular groove formed in the outer periphery of a shaft into a coupling hole in the central portion of a hub; plastic-deforming the vicinity of the hole in the end of a hub over the whole periphery; and causing material for the hub to plastic-flow so as to burry an recess of the shaft by the plastic-deformation and to apply compression stress to the shaft; whereby the coupling force is obtained by shearing force and compressing force provided by the material used for the hub, to couple the shaft with the hub. For achieving the second object, there is provided a method comprising: press-fitting an outer circumferential part having a plurality of annular grooves formed in the outer periphery of the shaft and being provided with a crest of a cylindrical part between the grooves into a coupling hole in the central portion of the hub; plastic-deforming the vicinity of the coupling hole in the end of the hub over the whole periphery; and causing material for the hub to plastic-flow so as to burry an recess of the shaft by the plastic-deformation and to apply compression stress to the shaft; whereby the coupling force is obtained by shearing force and compressing force provided by the material used for the hub, to couple the shaft with the hub. In this case, it is essential to adopt the press-fitting for coupling force.
According to the present invention as constructed above, even in coupling between a thin hub and a small diameter shaft, the satisfactory strength and accuracy, and sealing properties are obtained, and coupling can be attained with excellent productivity by inexpensive equipment.